a. Synoptic History
Hurricane Ivan was the first in a trio of hurricanes
to form and then remain over the eastern Atlantic Ocean during the latter part of September. The
wave from which Ivan developed was rather easily identified by its concentrated
area of thunderstorms seen on satellite pictures (not shown) over western Africa near the Greenwich Meridian on 14
September. Although deep convection diminished when the wave neared the coast, the residual cloudiness and sounding
data up through 700 mb from Dakar, Senegal showed the system's passage into the Atlantic on the 16th.
Strong convection associated with the wave redeveloped to the south of the Cape Verde Islands on the
17th. Position estimates from satellite analysts began that day. The cloudiness became more concentrated near the apex of
the wave on the 18th. Late that day, Dvorak T-numbers first reached 2.0 from the
NOAA TPC Tropical Analysis and Forecast Branch (TAFB) and 1.5 from the NOAA
Synoptic Analysis Branch (SAB). Slow development
followed and it is estimated that the system became a tropical depression near
0000 UTC on the 19th, about 175 n mi to the southwest of the Cape Verde Islands
(Fig. 1 [25K GIF] and Table 1).
The tropical cyclone was influenced by mid- to upper-level cyclonic flow centered to its northwest, over
the central Atlantic. At first, that pattern consisted of a trough elongated southward from 30N. During Ivan's
development, however, water vapor imagery showed the trough become a closed circulation that partially enveloped the
tropical cyclone. A second trough later affected Ivan as well. These features
influenced the details of Ivan's track (see Fig. 1), which was generally northwestward
at 10-15 kt from the 20th-25th.
The troughs also created an environment of vertical shear that led to Ivan's rather slow development
(Table 1). Ivan became a tropical storm late on the 20th
and during the following 48 hours wind speeds increased to around 55 kt as estimated from the
appearance of what could have been an eye on satellite pictures for about an hour.
Ivan's hurricane stage occurred rather far to the north. At 1400 UTC on the 23rd, the eye reappeared and
was more prominent than earlier, indicating that Ivan was becoming a hurricane as it neared 30N. After again
disappearing, the eye became its most distinct with a diameter of about 20 n mi, and Ivan is estimated to have reached its
peak intensity of about 80 kt on the morning of the 26th. The hurricane was then located
about 300 n mi to the west of the Azores Islands. At that time, the influence of the upper troughs on Ivan was decreasing
and the track of the hurricane was increasingly controlled by the westerlies just to the north. Late on the 26th, the eye
disappeared. The inner convective structure rapidly deteriorated while Ivan passed eastward over cooler waters to the north
of the Azores Islands on the 27th. Ivan was then becoming a weakening extratropical storm,
and then gale, which moved northeastward according to analyses issued in the United Kingdom.
b. Meteorological Statistics
The "best track" intensity was obtained from the data presented in
Figs. 2 (12K GIF) and 3 (13K GIF).
Those figures show Ivan's estimated central pressure and maximum one-minute wind speed, respectively, versus time.
Position and intensity estimates were obtained from analyses of satellite pictures by the
SAB and TAFB,
and by the Air Force Weather Agency (AFGWC in figures).
Analyses also included surface observations.
There were no reports of tropical storm force winds on land.
The ships listed below reported sustained tropical storm force winds. The reports of northerly or
northwesterly winds of 35-38 kt from the ship DACF near 16N 25W on 19 September
do not seem compatible with other ship reports in that vicinity, or with the analysis of the cyclone--centered a few hundred
nautical miles to the southwest at that time. In addition, the speeds shown for ship PJOX at 23/1800 UTC and 24/0300
UTC are probably double the actual winds based on analysis of data by the NWS Marine Prediction Center.
|Date/time (UTC)||Ship Id||Lat.(N)||Lon.(W)||Wind (kt)|
c. Casualty and Damage Statistics
Ivan is not known to have caused casualties or damages.
d. Forecast and Warning Critique
The average NHC track forecast errors were about 20% larger than the most-recent 10-year averages, but
were comparable in magnitude to most of the numerical model guidance (Table 2). The best
performance came from the NOGAPS model.
The NHC generally forecast an intensity for Ivan that was too low by about 10 kt at 24 hours and
20 kt at 72 hours.
Watches and warnings were not issued for Ivan, but NHC advisories did indicate the possibility that
tropical storm or hurricane force winds could spread over the Azores.